IQ imbalance image compensation in multi-carrier wireless communication systems

ABSTRACT

A method in a wireless communication terminal includes receiving an aggregated carrier including a first component carrier and a second component carrier, determining a level of interference from a signal received on the first component carrier to a signal on the second component carrier based on a signal characteristic of the first component carrier and a signal characteristic of the second component carrier, and providing signal interference information to a serving base station if the determined interference level satisfies a condition.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to wireless communications and,more particularly, to IQ imbalance image compensation in multi-carrierwireless communication systems.

BACKGROUND

In carrier aggregation (CA), a secondary serving cell (Scell) can bemuch stronger than a primary serving cell (Pcell), for example, based onreference signal received power (RSRP) measurements, due to radioresource management (RRM) inefficiencies. This is especially likely inCA scenarios with staggered beam patterns and non-overlapping coverageareas where approximately 5-10% of the UEs may have a Scell that isstronger than a Pcell by 10 dB (in RSRP) or more per R4-103677. Due toIQ imbalance, this power imbalance can result in a Scell imageinterfering with the Pcell signal when a wideband transceiver is used toreceive the Pcell and the Scell simultaneously, e.g., two 10 MHzadjacent carriers being aggregated (intra-band CA). With an IQ gainimbalance of 1.1 (approximately 26 dB adjacent channel interferencerejection ratio), a 10 dB RSRP delta between the Pcell and the Scell canlead to the Pcell signal to noise (SNR) being limited to around 15 dBthereby limiting the schedulable modulation coding scheme (MCS) on thePcell, e.g., a modulation coding scheme (MCS) 64 QAM rate=⅚ cannot bescheduled without performance degradation, as described in R4-104310.

In LTE Rel-10, up to 5 component carriers (CCs) can be aggregated (e.g.,5×20 MHz in a 100 MHz band) in the intra-band CA case. Although thegain/phase imbalance at the local oscillator (LO)/mixer is independentof the low-noise amplifier (LNA) filter bandwidth or theanalog-to-digital (ADC) bandwidth (BW) and the sampling rate, the amountof leakage of one CC into another depends on

-   -   (i) whether or not the receiver uses a single Fast Fourier        Transform (FFT) or multiple FFTs, and    -   (ii) whether or not there are filters that follow the ADC to        separate out the individual CCs in the multiple FFT case.        For the simplest case of two CCs (i.e., a Pcell and one Scell),        a multiple FFT receiver architecture is shown in FIG. 1 where        Component Carrier #1 (CC1) is received on a lower carrier        frequency relative to Component Carrier #2 (CC2). A single FFT        architecture is shown in FIG. 2.

For a receiver architecture the image from a transmission received onone CC interfering with another CC must be calibrated in CC-pairs (e.g.,for 5 CCs in intra-band CA, there are

$\left. {\begin{pmatrix}5 \\2\end{pmatrix} = {{{5!}/\left( {{2!}{3!}} \right)} = {10\mspace{14mu}{pairs}}}} \right).$This entails significant effort for the User Equipment (UE)manufacturer. Further, the filter characteristics might change as afunction of the number of activated/configured CCs and the bandwidth(BW) of each CC, possibly rendering pre-calibration infeasible.

The various aspects, features and advantages of the invention willbecome more fully apparent to those having ordinary skill in the artupon careful consideration of the following Detailed Description thereofwith the accompanying drawings described below. The drawings may havebeen simplified for clarity and are not necessarily drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art receiver architecture having multiple Fast FourierTransforms (FFTs).

FIG. 2 is a prior art receiver architecture having a single FFT.

FIG. 3 is an exemplary wireless communication system employing carrieraggregation.

FIG. 4 illustrates a process flow diagram.

FIG. 5 illustrates an exemplary coordination scheme for two componentcarriers.

FIG. 6 is a schematic illustrating I/Q imbalance image compensation.

FIG. 7 illustrates intra-band contiguous aggregated carrier.

FIG. 8 illustrates another process flow diagram.

DETAILED DESCRIPTION

In FIG. 3, a wireless communication system 100 comprises one or morefixed base infrastructure units 101, 102 forming a network distributedover a geographical region for serving remote units in the time and/orfrequency and/or spatial domain. A base unit may also be referred to asan access point, access terminal, base, base station, NodeB, enhancedNodeB (eNodeB), Home NodeB (HNB), Home eNodeB (HeNB), Macro eNodeB(MeNB), Donor eNodeB (DeNB), relay node (RN), femtocell, femto-node,network node or by other terminology used in the art. The one or morebase units each comprise one or more transmitters for downlinktransmissions and one or more receivers for uplink transmissions. Thebase units are generally part of a radio access network that includesone or more controllers communicably coupled to one or morecorresponding base units. The access network is generally communicablycoupled to one or more core networks, which may be coupled to othernetworks like the Internet and public switched telephone networks amongothers. These and other elements of access and core networks are notillustrated but are known generally by those having ordinary skill inthe art.

In FIG. 3, the one or more base units serve a number of remote units103, 104 within a corresponding serving area, for example, a cell or acell sector, via a wireless communication link. The remote units may befixed or mobile. The remote units may also be referred to as subscriberunits, mobiles, mobile stations, mobile units, users, terminals,subscriber stations, user equipment (UE), user terminals, wirelesscommunication devices, relay node, or by other terminology used in theart. The remote units also comprise one or more transmitters and one ormore receivers. In FIG. 3, the base unit 101 transmits downlinkcommunication signals to serve remote unit 103 in the time and/orfrequency and/or spatial domain. The remote unit 104 communicates withbase unit 102 via uplink communication signals. Sometimes the base unitis referred to as a serving or connected or anchor cell for the remoteunit. The remote units may also communicate with the base unit via arelay node.

In one implementation, the wireless communication system is compliantwith the 3GPP Universal Mobile Telecommunications System (UMTS) LTEprotocol, also referred to as EUTRA or 3GPP LTE or some later generationthereof, wherein the base unit transmits using an orthogonal frequencydivision multiplexing (OFDM) modulation scheme on the downlink and theuser terminals transmit on the uplink using a single carrier frequencydivision multiple access (SC-FDMA) scheme. The instant disclosure isparticularly relevant to 3GPP LTE Release 8 (Rel-8) and LTE Release 10(Rel-10) and possibly later evolutions, but may also be applicable toother wireless communication systems. More generally the wirelesscommunication system may implement some other open or proprietarycommunication protocol, for example, IEEE 802.16(d) (WiMAX), IEEE802.16(e) (mobile WiMAX), among other existing and future protocols. Thedisclosure is not intended to be implemented in any particular wirelesscommunication system architecture or protocol. The architecture may alsoinclude the use of spreading techniques such as multi-carrier CDMA(MC-CDMA), multi-carrier direct sequence CDMA (MC-DS-CDMA), OrthogonalFrequency and Code Division Multiplexing (OFCDM) with one or twodimensional spreading. The architecture in which the features of theinstant disclosure are implemented may also be based on simpler timeand/or frequency division multiplexing/multiple access techniques, or acombination of these various techniques. In alternate embodiments, thewireless communication system may utilize other communication systemprotocols including, but not limited to, TDMA or direct sequence CDMA.The communication system may be a Time Division Duplex (TDD) orFrequency Division Duplex (FDD) system.

E-UTRA systems also facilitate the use of multiple input and multipleoutput (MIMO) antenna systems on the downlink to increase capacity. Asis known, MIMO antenna systems are employed at the eNB through use ofmultiple transmit antennas and at the UE through use of multiple receiveantennas. A UE may rely on a pilot or reference symbol (RS) sent fromthe eNB for channel estimation, subsequent data demodulation, and linkquality measurement for reporting. The link quality measurements forfeedback may include such spatial parameters as rank indicator (RI), orthe number of data streams sent on the same resources; precoding matrixindex (PMI); and coding parameters, such as a modulation and codingscheme (MCS) or a channel quality indicator (CQI). Together MCS or CQI,PMI and RI constitute elements of the Channel State Information (CSI)which convey the quality of MIMO channel indicative of the reliabilityand condition number of the channel capable of supporting multi-streamcommunication between the eNB and the UE. For example, if a UEdetermines that the link can support a rank greater than one, it mayreport multiple CQI values (e.g., two CQI values when rank=2 bysignaling of the corresponding RI). Further, the link qualitymeasurements may be reported on a periodic or aperiodic basis, asinstructed by an eNB, in one of the supported feedback modes. Thereports may include wideband or subband frequency selective informationof the parameters. The eNB may use the rank information, the CQI, andother parameters, such as uplink quality information, to serve the UE onthe uplink and downlink channels. Spatial multiplexing transmission caneither be CRS-based (i.e., UE utilizes CRS for both CQI/PMI/RIestimation and for demodulation) or DRS-based (i.e., UE uses either CRSor CSI-RS for CQI/PMI/RI estimation and uses DRS for demodulation).

In an E-UTRA system, an uplink data channel may be a Physical UplinkShared Channel (PUSCH), an uplink control channel may be a physicaluplink control channel (PUCCH), a downlink control channel may be aphysical downlink control channel (PDCCH), and downlink data channel maybe a physical downlink shared channel (PDSCH). Uplink controlinformation may be communicated over the PUCCH and/or the PUSCH anddownlink control information is communicated typically over the PDCCH.The UE may further transmit uplink sounding reference signals to assistthe eNB on scheduling uplink (for frequency division duplex (FDD)) andfor one or both uplink and downlink for time-division duplex (TDD). Inthe Rel-8 LTE and beyond LTE systems such as Rel-10 (also known asLTE-Advanced), the base station transmits using an OFDM modulationscheme on the downlink and the UEs transmit on the uplink using a singlecarrier frequency division multiple access (SC-FDMA) scheme and/orDiscrete Fourier Transform Spread OFDM (DFT-SOFDM). On the UL, the UEmay transmit using contiguous or non-contiguous resource allocations andthe UE may also transmit data and control on the uplink simultaneouslyusing the so-called simultaneous PUCCH and PUSCH transmission scheme. Ina Frequency Division Duplex (FDD) operation, the frame structure in theuplink and downlink, each comprises of a 10 millisecond (ms) Radioframe, which is in turn divided into ten subframes each of 1 ms durationwherein each subframe is divided into two slots of 0.5 ms each, whereineach slot contains a number of OFDM symbols. The downlink and uplinkbandwidth are subdivided into resource blocks, wherein each resourceblock comprises of one or more subcarriers in frequency and one or moreOFDM symbols in the time domain (12 subcarriers×7 OFDM symbols fornormal Cyclic Prefix (CP)). In LTE resource blocks are defined on a slotbasis. A resource block (RB) is typical unit in which the resourceallocations are assigned for the uplink and downlink communications.Furthermore, the eNB configures appropriate channels for uplink anddownlink control information exchange. For the DL the physical downlinkcontrol channel (PDCCH) is used for sending the uplink and downlinkcontrol information to the UEs. The PDCCH is sent in the beginningportion of a subframe on a potentially variable number of OFDM symbols,and this number (typically 0 to 3 for large system bandwidths such as 5MHz, etc and 0 to 4 for smaller system bandwidths such as 1.25 MHz) issignaled on the Physical Control Format Indicator Channel (PCFICH) orsent via higher layer signaling. However, in other scenarios, the PDCCHmay also be located in certain fixed or variable time/frequency/spatialresources i.e., spanning one or more subcarriers in one or moresubframes and/or one or more spatial layers. For example, it may occupya subset of resource blocks instead of spanning the entire DL systembandwidth. The Physical Hybrid ARQ Channel (PHICH) is the Acknowledgmentindicator channel used to send the HARQ feedback on the DL for theuplink data transmissions from the UE. The PCFICH, PHICH, PDCCH are senton OFDM symbols at the beginning of the DL subframes. In some subframessuch as ABS or when the eNB has no UEs scheduled (i.e. very low or noload cases), these channels may be absent. In LTE Release-8, the masterinformation block (MIB) is sent on the Physical Broadcast CHannel(PBCH), the MIB comprises of system frame number (SFN), downlink systembandwidth, number of signaled downlink transmit antennas (or the numberof CRS ports), and Physical Hybrid ARQ Channel (PHICH) configuration(i.e. duration). In LTE Release-8, the PBCH is sent on subframe 0 (eachsubframe comprising of two slots, each slot corresponding to a 0.5milli-second). The Synchronization signals are transmitted on the innersix PRBs or inner 72 subcarriers (i.e. 1.25 MHz) on subframes 0 and 5.The exact location of the Synchronization signals depends upon theduplex type, Cyclic Prefix length, etc.

If an MBSFN subframe is configured, the subframe may contain an initialportion (near the beginning of the subframe) containing a unicast regionand the rest of the subframe may be configured differently based on thehigher layer signaling. If the MBSFN subframe is used for transmissionof multicast transmission channel (MTCH), then the rest of the subframemay contain multicast OFDM symbols with cyclic prefix (CP) that may bedistinct (and likely larger) than the CP used for the initialtransmission. In Rel-8/9, the MBSFN subframe configuration is typicallysent in the System Information Block (SIB) SIB2 message, wherein theSIB2 message is a higher layer message sent on the PDSCH by the eNB. Theschedule of SIB2, SIB3, (and other SIB messages) is indicated in SIM.The System information typically changes on the order of SIBtransmission window (e.g. in multiples of 8 ms) i.e. the systeminformation update only when a new SI-transmission window begins and theUEs are paged to indicate a SI update so that they can re-acquire systeminformation. Typically SIB transmissions are not allowed in MBSFNsubframes as there may be no CRS in the data region of the MBSFNsubframes.

In aggregated carrier systems, a User Equipment (UE) can receive andtransmit control and data signaling on multiple component carriers(CCs). Initially, the UE may communicate with the network by receivingonly a single CC (Primary or Anchor CC). In some implementations, thenetwork sends a configuration message (SI configuration message) to theUE on the primary CC with system information (SI) corresponding to otherCCs on which the network may schedule the UE. The SI typically includesCC specific information that the UE is required to store in order tocommunicate with the network on other CCs. The SI can include CCspecific information such as CC carrier frequency, downlink (DL)bandwidth, number of antennas, downlink reference signal power, uplink(UL) power control parameters and other information that does not changefrequently. In some AC systems, base station sends the SI configurationmessage to the UE using Radio Resource Configuration (RRC) signaling,since the SI does not change frequently and the payload associated withthe SI configuration is relatively large. Upon receipt of the SIconfiguration, the UE stores the SI for other CCs but continues tocommunicate with the network by only receiving the primary CC. The otherCCs for which the UE has received SI and the primary CC constitute theUE's “configured CC set”.

According to one aspect of the disclosure, illustrated in process flowdiagram 400 in FIG. 4, in a wireless communication terminal receives anaggregated carrier including a first component carrier and a secondcomponent carrier, as indicated at 410, from one or more base stationsin the wireless communication system. The first component carriergenerally comprises a reference signal (RS). In one embodiment, thereference signal is embodied as a cell-specific reference signal (CRS),a positioning reference signal (PRS), UE reference signal (UE-RS),Channel State Information Reference Signal (CSI-RS), a Demodulationreference signal (DRS or DM-RS), a pilot signal, a beacon signal amongother reference symbols or signals that may be used as a basis fordetermining a leakage from the first component carrier to the secondcomponent carrier as described more fully below.

In FIG. 4, at 420, the UE measures leakage of the reference signal fromthe first component carrier onto the second component carrier. In oneimplementation, the reference signal is transmitted on a particular timeslot, for example on the first component carrier in the exampledescribed above. According to this example, the reference signal leakagefrom the first component carrier is measured on a corresponding timeslot on the second component carrier. Generally the time slots on thefirst and second component carriers are coincident in the time domain,although in some circumstances they may be only partially aligned oroverlapping in the time domain.

In one embodiment, pilots transmitted on one CC can be used to estimatethe component of the image on a second CC. This can be accomplished bycoordinating transmission on the two CCs where transmit/blank time slotsor zones are created. FIG. 5 illustrates an exemplary coordinationscheme for two CCs, where the eNB blanks transmission on CC2 when CC1transmits only pilots on one time slot and the eNB blanks CC1 when CC2transmits only pilots in a second time slot. In LTE, a blank slot can becreated by configuring a MBSFN subframe and not scheduling PDSCH in thatsubframe (and as a result CRS is also absent in such a subframe) suchthat the blank slot is equivalent to the entire non-control region. Atransmit slot can be created by configuring a normal subframe that doesnot carry any PDSCH (but, by definition carries at least the CRS). CRSfrom only one antenna port may be transmitted to assist imagecompensation. Image compensation can be summarily described as follows.Receiving a RS transmission on the first CC results in a leakagecomponent on the second CC due to receiver non-linearity. The leakagecomponent is a function of both the received RS (with known signalstructure) and some variables that parameterize the receivernon-linearity (in this case, IQ gain and phase imbalance which is alsoknown as the quadrature non-linearity). The received signal on thesecond CC and the known RS sequence are made use of to estimate thevariables that parameterize the receiver non-linearity. A detailedembodiment of this procedure is described under “Training Period” and“Parameter Estimation” below. Once the parameters associated with thereceiver non-linearity are determined, a compensation circuit can makeuse of these parameters to pre-compensate the signal received on thesecond CC before the signal is processed to extract data embedded in thesignal. In one embodiment, the compensation circuit can estimate theleakage signal component that is corrupting the portion of the signalcorresponding to the second CC on a real-time basis and subtract it offprior to FFT processing as shown in FIG. 6. Instead of using CRS (withor without transmit/blank coordination), the positioning referencesignal (PRS) can be configured to assist UE in IQ imbalancecompensation. Six PRS patterns are available (v_shift=mod(PCID, 6))together with time-domain PRS muting which leads to a largetime-frequency reuse factor.

In one implementation, the leakage of the reference signal from thefirst component carrier onto the second component carrier is measuredonly intermittently, for example, during a time interval that may beconsidered a training period. The measurement or estimation of theleakage may also be made periodically, for example, every fewmilliseconds or every second or so. In another embodiment, the UEreceives scheduling information from the base station indicating whenthe reference signal will be transmitted on the first component carrier.In one particular implementation, the scheduling information isindicative of a time offset and in some embodiments a periodicity forthe reference signal. For example, the time offset may be indicative ofwhen the reference signal will be received relative to some reference,e.g., the beginning of a frame or subframe, and the periodicity mayindicate how frequently the reference signal is transmitted. The UE neednot measure leakage on every transmission of the reference signal. Forexample, the frequency with which the leakage is measured may bepredicated on some criteria, for example, a metric indicative of motionor velocity of the UE or some other metric indicative of a need forupdating the leakage measurement.

In FIG. 4, at 430, the UE receives a signal on the aggregated carrier,for example, on the first and second component carriers described above.At 440, the UE compensates for leakage of the signal from the firstcomponent carrier to the second component carrier based on themeasurement of the leakage of the reference signal.

In one implementation, generally, the UE generates or estimates one ormore parameters based on the measurement of the leakage of the referencesignal. In the example, above this leakage occurs from the firstcomponent carrier to the second component carrier. The UE compensatesfor the leakage by applying a transformation of the parameter to thereceived signal. The transformation may be in the time domain or in thefrequency domain. In one embodiment, the transformation of the parameterto the received signal is a linear combination of the received signalscaled by a first coefficient and a complex conjugate of the receivedsignal scaled by a second coefficient, and wherein the first and secondcoefficients of the linear combination are a function of the estimatedparameter. FIG. 6 illustrates an implementation where the parametersassociated with the image of CC1 are estimated (post-FFT and based on asignal template of pilot transmission in CC1) and are used forcompensating for the CC1's image in time-domain (prior to FFT) in CC2'sreceive chain. This is applicable to both single-FFT and multiple-FFTarchitectures illustrated in FIGS. 1 and 2.

In some LTE releases, the carrier separation for intra-band CA ismultiple of 300 kHz (i.e., the least common multiple of raster spacing100 kHz and subcarrier spacing 15 kHz). This allows the use of a singleFFT receiver for demodulating multiple aggregated CCs within the sameband. The simplest case of 10 MHz+10 MHz intra-band contiguous CA isshown in FIG. 7.

A method of estimating the parameters associated with a Scell imagearising out of IQ imbalance is described below. Consider the case of 2CCs of equal BW on adjacent carriers with a separation of 2KΔf, where Δfis the subcarrier spacing and K is an integer compliant with the carrierspacing requirements in R4-104825. Assuming that the carrier center isat about the mid-point of the transmission BW, the received pre-FFTsignal can be written asx(t)=e ^(j(K+1)Δf−t) x ₁(t)+e ^(−jKΔf−t) x ₂(t),

where x₁(t) and x₂(t) represent the time-domain signals for CC#1 andCC#2 respectively.

Due to IQ imbalance, the received signal can be written asy(t)=αx(t)+β x (t),

where α=cos φ+jε sin φ and β=ε cos φ−j sin φ, and φ is the LO phaseimbalance and ε is deviation of the gain from unity representing the LOgain imbalance. Therefore, y(t) can be written asy(t)=e ^(j(K+1)Δf−t)(αx ₁(t)+βe ^(−jΔf−t) x ₂(t))+e ^(−jKΔf−t)(αx₂(t)+βe ^(−jΔf−t) x ₁(t)).

Post-FFT, the frequency domain signal for the two CCs have the followingequivalent representation.CC#1:Y ₁(k)=αX ₁(k)+β X ₂(N−k−1)CC#2:Y ₂(k)=αX ₂(k)+β X ₁(N−k−1)

where k=0, 1, . . . , N−1 and N is the number of subcarriers in each CC.Clearly, the second CC leaks into the first and vice-versa. The image isproportional to the complex-conjugate of the signal reversed in sequencein the frequency-domain and shifted by one subcarrier. If uncompensated,this image term can limit the achievable SNR in each CC.

In a time slot where only CRS is being transmitted on both CCs, theremay or may not be CRS interference from one CC (image) to CRS receptionon a second CC depending on relative frequency shifts for CRS and thenumber of transmit antenna ports. For example, for 2 Tx, there is no CRScollision if mod(PCID₁,3) is equal to mod(PCID₂,3) where PCID₁ and PCID₂are the PCIDs associated with the two CCs. In this case, there is noneed to blank transmission on one CC when the other CC is transmittingCRS as shown in FIG. 5. Instead, both CRS can be transmitted on both CCsin CRS-only subframes (e.g., almost blank subframe configured on anormal subframe where PDSCH is not transmitted). Muting transmission onall CRS ports except for one (e.g., port #0) can further reduce thelikelihood of CRS collision.

In a single-FFT receiver, if N_(FFT) is the basic FFT size associatedwith each CC, a FFT of size 2N_(FFT) can be used for extracting signalsfor the two CCs in the present embodiment. In a dual-FFT receiver, thesignal after the ADC stage is bandpass filtered prior to FFT to extracta signal corresponding to the two CCs in an alternate embodiment. Aunified approach can be developed for estimating α and β for both thesingle and dual FFT receivers (i.e., both embodiments) as the responseof bandpass filter in the multiple FFT case is known for eachimplementation.

Estimating the parameters α and β based on CRS transmission in one orboth CCs can constitute the first step in IQ imbalance compensation.After estimating these parameters, a correction can be carried out intime domain (pre-FFT) for both single-FFT and dual-FFT implementationsas shown in FIG. 6 by making use of the equation:

${{z_{corr}(t)} = \frac{{\overset{\_}{\alpha}\;{z(t)}} - {\beta\;{\overset{\_}{z}(t)}}}{{\alpha }^{2} - {\beta }^{2\;}}},$

where z(t) is the time-domain signal for a single CC (dual-FFT receiver)or all CCs (single-FFT receiver).

When there is no CRS collision (either due to the selection of suitablePCID pairs for the two CCs or due to transmit/blank coordination of FIG.3), the received signal can be divided into four groups of subcarriersover which CRS is received:H ^((1,1))(l):=Y ₁(k) s ₁(k),kεS _(1,1)H ^((1,2))(l):=Y ₁(N−k−1) s ₁(N−k−1),kεS _(1,2)H ^((2,1))(l):=Y ₂(k) s ₂(k),kεS _(2,1)H ^((2,2))(l):=Y ₂(N−k−1) s ₂(N−k−1),kεS _(2,2)

where S_(i,j) is the set of subcarriers over which CRS transmission onCC#j is received on CC#i, and s_(j)(k) is the CRS sequence for CC#j infrequency domain. The index l belongs to the range l=0, 1, . . . , L−1,where L=100 for the 10 MHz case. The index l maps to index k in setS_(Z) in a one-to-one fashion.

If H^((j))(l) is the actual channel response on CC#j (i.e., noise-lesschannel response or ideal channel estimate), we have the following setsof equations.H ^((1,1))(l)=αH ⁽¹⁾(l)+noiseH ^((1,2))(L−l−1)= βγ_(L−1) H ⁽²⁾(l)+noiseH ^((2,1))(l)=αH ⁽²⁾(l)+noiseH ^((2,2))(L−l−1)= βδ_(L−1) H ⁽¹⁾(l)+noise

where γ_(l)'s are the (known) filter response coefficients for receivinga signal on CC#2 in the BW of CC#1 and δ_(l) is the corresponding filterresponse for receiving signal on CC#1 in the BW of CC#2. For thesingle-FFT case, γ_(l)=δ_(l)=1 as there is no bandpass filtering. Forthe multiple FFT case, these coefficients are knownimplementation-dependent constants.

The sequence of H^((j))(l)'s can be written in vector form as

$\begin{bmatrix}{H^{(j)}(1)} \\\vdots \\{H^{(j)}(L)}\end{bmatrix} = {W_{L \times P}^{(j)}\begin{bmatrix}{h(1)} \\\vdots \\{h(P)}\end{bmatrix}}$

where h(n) are the time-domain channel response coefficients and W_(L×P)^((j)) is the subsampled matrix of a 2N_(FFT)×2N_(FFT) DFT matrixapplicable to the reception of CRS subcarriers in CC#j, and P is thelength of the channel response at sample rate 2N_(FFT).

In matrix form, the equations above can be written as

$\begin{matrix}{\underset{\underset{z_{1}}{︸}}{\begin{bmatrix}{H^{({1,1})}(0)} \\\vdots \\{H^{({1,1})}\left( {L - 1} \right)} \\{{\overset{\_}{H}}^{({2,2})}\left( {L - 1} \right)} \\\vdots \\{{\overset{\_}{H}}^{({2,2})}(0)}\end{bmatrix}} = {{{\underset{\underset{F{({\alpha,\beta})}}{︸}}{\begin{bmatrix}{\alpha\; I_{L}} & O \\O & {\overset{\_}{\beta}\; I_{L}}\end{bmatrix}}\begin{bmatrix}G \\D\end{bmatrix}}W_{L \times P}^{(1)}\underset{\underset{h}{︸}}{\begin{bmatrix}{h(0)} \\\vdots \\{h\left( {P - 1} \right)}\end{bmatrix}}} + {noise}}} & \; \\{\underset{\underset{z_{2}}{︸}}{\begin{bmatrix}{H^{({1,2})}(0)} \\\vdots \\{H^{({1,2})}\left( {L - 1} \right)} \\{{\overset{\_}{H}}^{({2,1})}\left( {L - 1} \right)} \\\vdots \\{{\overset{\_}{H}}^{({2,1}\;)}(0)}\end{bmatrix}} = {{{\underset{\underset{F{({\alpha,\beta})}}{︸}}{\begin{bmatrix}{\alpha\; I_{L}} & O \\O & {\overset{\_}{\beta}\; I_{L}}\end{bmatrix}}\begin{bmatrix}G \\D\end{bmatrix}}W_{L \times P}^{(2)}\underset{\underset{h}{︸}}{\begin{bmatrix}{h(0)} \\\vdots \\{h\left( {P - 1} \right)}\end{bmatrix}}} + {noise}}} & \;\end{matrix}$

where G=diag({γ_(l)}) and D=diag({δ_(l)}).

The parameters (α, β) and the channel response h can be jointlyestimated by the minimization problem:

$\min\limits_{({\alpha,\beta,h})}{{{\begin{bmatrix}z_{1} \\z_{2}\end{bmatrix} - {{{\left( {I_{2} \otimes {F\left( {\alpha,\beta} \right)}} \right)\begin{bmatrix}G & O \\D & O \\O & G \\O & D\end{bmatrix}}\begin{bmatrix}W_{L \times P}^{(1)} \\W_{L \times P}^{(2)}\end{bmatrix}}h}}}^{2}.}$

FIG. 6 illustrates an implementation where parameters associated withthe image of CC1 are estimated (post-FFT and based on signal template ofpilot transmission in CC1) and are used for compensating for the CC1'simage in time-domain (prior to FFT) in CC2's receive chain. This isapplicable to both single-FFT and multiple-FFT architectures.

For estimation of the parameters (α,β), we show a derivation that usesCRS observations for only one transmit antenna port. For 2/4 Tx, we canaugment CRS observations for the other transmit antenna ports in thecalculation of the parameters (α, β).

The time-domain compensation for IQ imbalance as shown in Appendix A canbe applied on a per-receive antenna basis. This method is thereforeapplicable to multi-layer transmissions.

The UE generally comprises a controller coupled to a wirelesstransceiver wherein the controller is configured to cause the UE toperform the various functions described herein including receiving theaggregated carrier, measuring leakage of the reference signal, receivinga signal on the aggregated carrier, and compensating for the leakingamong the other functionality described herein. The methods andfunctions described herein may be performed by a digital processorexecuting software of firmware residing in a memory device.Alternatively the functionality of the UE may be performed by equivalenthardware or by a combination of hardware and software. Similarly, theeNB also comprises a controller coupled to a wireless transceiverwherein the controller is configured to cause the eNB to perform thevarious functions described herein including transmitting schedulinginformation indicating when the reference signal will be transmitted,transmitting the reference signal on at least one of the indicated timeslots, and blanking a data signal transmission on a second componentcarrier on a time slot that at least partially overlaps in time with thetime slot where the reference signal is transmitted on the firstcomponent carrier among other functionality provided herein.Alternatively the functionality of the eNB may be performed byequivalent hardware or by a combination of hardware and software.

According to one aspect of the disclosure, illustrated in process flowdiagram 800 in FIG. 8, the wireless communication terminal receives anaggregated carrier including a first component carrier and a secondcomponent carrier, as indicated at 810, from one or more base stationsin the wireless communication system. At 820, the UE determines a signalcharacteristic of the first component carrier and a signalcharacteristic of the second component carrier. The signalcharacteristic of the component carriers is selected from a groupcomprising a reference signal received power (RSRP), a reference signalstrength indicator (RSSI), a total received power, a channel qualityindicator (CQI), and a hypothetical block error rate (BLER) applicableto a packet-coded transmission.

In FIG. 8, at 830, the UE determines a level of interference from asignal received on the first component carrier to a signal on the secondcomponent carrier based on the signal characteristic of the firstcomponent carrier and the signal characteristic of the second componentcarrier. In one embodiment, the determines a reference signal receivedpower (RSRP) of the first component carrier and a reference signalreceived power (RSRP) of the second component carrier. The level ofinterference may then be determined based on a ratio of the RSRP of thefirst component carrier with respect to the RSRP of the second componentcarrier. Alternatively, the level of interference may be determined bycomparison of a difference of a logarithm of the RSRP of the first andsecond carrier components and comparison of the difference to athreshold.

In FIG. 8, at 840, the UE provides signal interference information to aserving base station if the determined interference level satisfies acondition. Satisfaction of the conditions may be determined bycomparison of the ratio to a threshold. In one embodiment, the UEindicates the signal interference information to the base station onlywhen both the determined interference level satisfies the condition andupon determining that a modulation coding scheme scheduled on the secondcomponent carrier is subject to a specified level of interference fromthe first component carrier, as described more fully below. In someinstances, the UE receives a primary serving cell change command fromthe serving base station in response to sending the signal interferenceinformation to the serving base station.

The UE generally comprises a controller coupled to a wirelesstransceiver wherein the controller is configured to cause the UE toperform the various functions described herein including receiving theaggregated carrier, measuring leakage of the reference signal,determining signal characteristics on the component carriers,determining the level of interference, and providing signal interferenceinformation to the base station among the other functionality describedherein. The methods and functions may be performed by a digitalprocessor executing software of firmware residing in a memory device.Alternatively the functionality of the UE may be performed by equivalenthardware or by a combination of hardware and software. Similarly, theeNB also comprises a controller coupled to a wireless transceiverwherein the controller is configured to cause the eNB to perform thevarious functions described herein including receiving and processingmessages from the UE indicating that an impairment from a signaltransmitted on a first component carrier to a signal transmitted on thesecond component carrier exceeds a certain level, configuring a thirdcomponent carrier as a new primary cell component carrier, and sending auser-specific message to the wireless terminal indicating that the thirdcomponent carrier has been configured as the new primary cell componentcarrier, among other functionality provided herein. Alternatively thefunctionality of the eNB may be performed by equivalent hardware or by acombination of hardware and software.

In a more specific implementation, the second component carrier isconfigured as a primary serving cell and the first component carrier isconfigured as a secondary serving cell. In response to receiving aprimary serving cell change command from the serving base station, theUE configures a third component carrier as a new primary serving cell.In one use case, the first component carrier is configured as theprimary serving cell and the second component carrier is configured asthe secondary serving cell. According to this configuration, the thirdcomponent carrier corresponds to the first component carrier.

In another scenario, the UE receives a DCI grant indicating transmissionof a sequence of information symbols according to a modulation order, acoding scheme on the second component carrier and a transmission modesuch one of a single-layer transmission, a spatial multiplexing method,a transmit diversity method. In EUTRA Rel-10, if spatial multiplexingtransmission scheme is used, the transmission can be based either on CRSor on DRS (and the transmission rank associated with the transmissioncan be equal to one of 1, 2, 3, 4 for CRS-based transmission and equalto one of 1, 2, . . . , 8 for DRS-based transmission). According to thisscenario, the UE also determines a level of interference that thesequence of information symbols is subject to from the first componentcarrier based on the signal characteristic of the first componentcarrier and the signal characteristic of the second component carrier.The UE then indicates the signal interference information to the basestation only if the level of interference satisfies a condition asdescribed above.

Consider an LTE example where the Scell RSRP exceeds the Pcell RSRP by11 dB and both the first and second component carriers have full PDSCHloading. Also assume that all resource elements (REs) in a subframe haveequal power. If the UE's Scell-to-Pcell adjacent channel interferencerejection ratio (ACIRR) which is indicative of the receiver's adjacentchannel suppression capability is 26 dB, the SINR on the Pcell is 15 dB.This means that only some MCSs, e.g., MCS 16-28 are degraded if a PDSCH(w.r.t. to BLER) with such an MCS were to be scheduled for the UE. Ifthe UE is not scheduled at all or if a lower MCS such as MCS 1-15 isscheduled, there is no problem. So, the UE sends an indication to theeNB identifying that there is a problem only when the condition is met(i.e., when Scell RSRP minus Pcell RSRP>10 dB), and when the UE receivesa DCI grant indicating that a MCS belonging to a certain range (MCS16-28 in this example) is likely to have degraded performance has beenscheduled. The degradation of PDSCH w.r.t. BLER can also be function ofthe transmission scheme used. For example, the BLER degradation can bemore severe for higher rank transmission. Therefore, in an alternateembodiment, the UE can send an indication only when both an MCS within apre-determined range is scheduled and when the spatial multiplexingtransmission rank exceeds a pre-determined value.

In response to receiving such an indication fro the UE, the eNB canschedule a lower MCS or a transmission with lower rank if the effectivethroughput is increased by lowering the MCS or the transmission rank. Itis possible that the packet reception loss associated with the currentMCS/rank is small and lowering the MCS can reduce the effectivethroughput, in which case, the eNB does not lower the MCS ortransmission rank.

UE-measured CQI can track SINR degradation due to I/Q imbalance inconstant loading or slowly-varying conditions. But, there are somescenarios where this may not be possible particularly when the loadingon the Scell is bursty. The leakage due to I/Q gain/phase imbalance is afunction of Scell PDSCH loading. Assuming linear dependence (i.e.,leakage level on Pcell due to I/Q gain/phase imbalance=constant timesScell loading factor), the leakage level on the Pcell due to I/Qgain/phase imbalance can vary over a 10*log10(12×12/(4×4))=9.5 dB rangewhich could significantly impact the SINR and therefore, the measuredCQI. Therefore, there might be a non-trivial probability of UEover-reporting the CQI. The eNB scheduler may take into account the NACKrunning average (i.e., UE actual decoding error rate) in addition to thereported to CQI in its scheduling decisions. But, as describedpreviously in one of the embodiments, the RSRP difference between thePcell and Scell can be used to detect a potential leakage problem asRSRP remains invariant to the loading conditions on the Scell. This canthen be used to alert the eNB against possible over-scheduling of MCS.

Several UE-based triggers for mitigating the IQ imbalance problem arediscussed below. These methods are relevant mainly to single-FFTreceivers that are likely to have limited IQ imbalance compensationcapability. In one embodiment, the UE can detect that its Pcell is beingdesensed by the Scell image when the RSRP difference between the Pcelland Scell is large. The UE's image rejection capability, which is areceiver-specific implementation property, can be calibrated. Since theUE monitors Pcell RSRP and Scell RSRP on a continuous basis (both whenin DRX and in non-DRX), it can determine whetherRSRP(Scell)−RSRP(Pcell)>threshold_(—) A.

In an alternate embodiment, the UE can determine whetherRSSI(Scell)−RSRP(Pcell)>threshold_(—) BORRSSI(Scell)−RSSI(Pcell)>threshold_(—) C.

The UE can send an indication to the serving eNB (say, over Pcell UL)that Pcell downlink (DL) is likely to get desensed by the Scell image.The quantity threshold_A can be a pre-calibrated threshold determined,for example, based on receiver gain/phase imbalance by the UEmanufacturer. The eNB can use this report as a trigger to re-configurethe UE's Pcell. For example, the strongest Scell can be configured as anew Pcell for the UE. The UE can also report a Scell index or identifierto indicate which Scell is or may be desensing the Pcell. As a furtherrestriction to reporting, the UE can send such a report only when a MCSwith sufficiently high modulation order/code rate combination that islikely to be impacted by the desense problem is being scheduled,otherwise the UE need not send a report.

In one embodiment, the UE sends a request to the eNB asking that acertain Scell be configured as the new Pcell. In an alternativeembodiment, the UE can determine (e.g., based on RSSI measurements onthe Scell) that the Scell is heavily loaded, in which case the UE canrequest the eNB to perform an inter-frequency handover to a new Pcell.In another alternative embodiment, the UE can request the eNB that theScell be activated (if it is currently deactivated) and the UE can bescheduled only on a Scell, wherein scheduling is performed either byPDCCH on the Scell or by means of inter-CC scheduling where PDCCH issent on the Pcell.

In some embodiments, the UE can also advertise its image rejectioncapability. For example, the UE can send Adjacent Channel InterferenceRejection Ratio (ACIRR) on its uplink (UL) to the eNB. This will enablethe eNB to determine if there is an image problem based on RSRP reportsthat the UE sends on a periodic basis.

According to another aspect of the disclosure, a wireless communicationterminal receives a request from a serving base station for wirelesscommunication terminal capability information. In response, the UE sendscapability information to the serving base station, wherein thecapability information includes an Adjacent Channel InterferenceRejection Ratio (ACIRR) for at least one carrier frequency (e.g.,EARFCN) pair as part of the RRC capability information exchange. In oneembodiment, the UE sends ACIRR together with either a band combinationor a carrier aggregation bandwidth class. According to this embodiment,the UE sends separate ACIRR values for each band combination, eachcarrier aggregation bandwidth class, etc. In a further alternativeembodiment, instead of sending the ACIRR value for a EARFCN pair, the UEcan send the ACIRR values at different frequencies possibly in relationthe different serving cell component carrier frequencies (e.g., If Pcellat carrier frequency F1, ACIRR corresponding to Scell #1 at carrierfrequency F2, Scell #2 at carrier frequency F3, etc. are sent by the UEto the eNB).

Although, the specific embodiments presented are applicable tomitigation of receiver impairment due to LO IQ gain/phase imbalance,these techniques can be readily extended to the compensation of otherreceiver impairments such as phase noise and carrier frequency offset.The effect of receiver impairments on the received signal, in manycases, can be parameterized by a few variables which can be estimated bysuitable processing of the received RS sequence. A compensation circuitthat makes use of the estimated parameters can be designed to mitigatethe effect of the receiver impairment in the received signal.

While the present disclosure and the best modes thereof have beendescribed in a manner establishing possession and enabling those ofordinary skill to make and use the same, it will be understood andappreciated that there are equivalents to the exemplary embodimentsdisclosed herein and that modifications and variations may be madethereto without departing from the scope and spirit of the inventions,which are to be limited not by the exemplary embodiments but by theappended claims.

What is claimed is:
 1. A method in a wireless communication terminal,the method comprising: receiving, at the terminal, an aggregated carrierincluding a first component carrier and a second component carrier;determining a signal characteristic of the first component carrier and asignal characteristic of the second component carrier; determining alevel of interference from a signal received on the first componentcarrier to a signal on the second component carrier based on the signalcharacteristic of the first component carrier and the signalcharacteristic of the second component carrier; determining whethermodulation coding scheme scheduled on the second component carrier issubject to a specified level of interference from the first componentcarrier based on determining that the modulation coding scheme scheduledon the second component carrier is within a pre-determined range; andproviding signal interference information to a serving base station onlywhen the determined interference level exceeds a predetermined value andwhen the modulation coding scheme scheduled on the second componentcarrier is subject to the specified level of interference.
 2. The methodof claim 1, further comprising: determining a reference signal receivedpower (RSRP) of the first component carrier and a reference signalreceived power (RSRP) of the second component carrier, determining aratio of the RSRP of the first component carrier with respect to theRSRP of the second component carrier; determining the level ofinterference based on the ratio.
 3. The method of claim 1, wherein thesignal characteristic of the first and second component carriers isselected from a group consisting of: a reference signal received power(RSRP), a reference signal strength indicator, a total received power, achannel quality indicator, and a block error rate applicable to apacket-coded transmission.
 4. The method of claim 1, wherein the secondcomponent carrier is configured as a primary serving cell and the firstcomponent carrier is configured as a secondary serving cell, the methodfurther comprising: receiving a primary serving cell change command fromthe serving base station in response to sending the signal interferenceinformation to the serving base station; and configuring a thirdcomponent carrier as a new primary serving cell in response to receivingthe command.
 5. The method of claim 1, wherein the first componentcarrier is configured as a primary serving cell and the second componentcarrier is configured as a secondary serving cell.
 6. A method in a basestation configured to transmit an aggregated signal that includes atleast two component carriers, the method comprising: receiving a messagefrom a wireless terminal being served by the base station indicatingthat an interference from a signal transmitted on a first componentcarrier to a signal transmitted on the second component carrier exceedsa certain level due to receiver impairment in the wireless terminal,wherein the second component carrier is a secondary cell componentcarrier and the first component carrier is a primary cell componentcarrier; wherein the wireless terminal determining whether modulationcoding scheme scheduled on the second component carrier is subject to aspecified level of interference from the first component carrier basedon determining that the modulation coding scheme scheduled on the secondcomponent carrier is within a pre-determined range; wherein the wirelessterminal transmitting the message indicating the interference to thebase station only when the interference from the signal transmitted onthe first component carrier to the signal transmitted on the secondcomponent carrier exceeds said certain level and when the modulationcoding scheme scheduled on the second component carrier is subject tothe specified level of interference; and configuring a third componentcarrier as a new primary cell component carrier in response to receivingthe message.
 7. The method of claim 6, further comprising sending auser-specific message to the wireless terminal indicating that the thirdcomponent carrier has been configured as the new primary cell componentcarrier.
 8. The method of claim 6, wherein the third component carrierand the second component carrier correspond to the same componentcarrier, the method further comprising-configuring the second componentcarrier as the new primary cell component carrier in response toreceiving the message.
 9. The method of claim 6, wherein the messageindicating that the impairment from the signal transmitted on the firstcomponent carrier to the signal transmitted on the second componentcarrier exceeds said certain level includes a signal characteristic ofthe first component carrier and a signal characteristic of the secondcomponent carrier.
 10. The method of claim 9, wherein the signalcharacteristic of the first and second component carriers is selectedfrom a group consisting of: a reference signal received power, areference signal strength indicator, a total received power, a channelquality indicator, and a block error rate (BLER) applicable to apacket-coded transmission.
 11. The method of claim 6, wherein theimpairment from the signal transmitted on the first component carrier tothe signal transmitted on the second component carrier is due to atleast one of a I/Q gain and phase imbalance impairment, a phase noiseimpairment or a carrier offset impairment.